1, the balance of WC - CO alloy sintering process, high purity, when the mixture and achieve ideal uniformity, and sintering temperatures extremely slow, you can think sintering sintering in a balanced state of sintering: (theory, under the condition of normal production does not exist)
(1) in the process of slow warming, WC slowly dissolve to form a solid solution in the cobalt, its solubility increases with temperature rise. When the temperature reaches the eutectic temperature 1340 ℃, began to appear eutectic composition of sintered body liquid, and at the same time, the WC melting, liquid quantity increases unceasingly, the eutectic temperature keep long enough, let sintered body formed undissolved solid phase (WC) and liquid phase equilibrium. Continue to slowly heat up, liquid continues to increase. Such as the temperature to 1400 ℃, the liquid phase is about 47% (weight).
(2) alloy balance heating and cooling process instead. When alloy cooling slowly from the 1400 ℃, the first at the beginning of exhalation WC crystal from the liquid phase. When the temperature dropped to eutectic reaction during eutectic temperature and precipitation in liquid phase WC crystal, eutectic reaction, liquid disappear, into the solid phase transformation, and with the temperature decrease from the solid solution precipitation redundant WC crystal three times. When the temperature below 900 ℃, the solid solution of WC content lower than 1.0%, alloy organization for early (WC original + WC + WC eutectic crystal + WC three + Y + CO) solid solution, alloy phase composition of WC + Y two-phase alloy.
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2, the imbalance of sintering: because it is difficult to achieve ideal mixture uniformity, and sintering process to heat up quickly, so actual sintering for unbalanced sintering:
(1) because of thermal hysteresis, only will appear above the eutectic temperature liquid phase;
(2) in a small region in the sintered body in liquid in different time;
(3) without enough time of heat preservation, the composition of liquid phase sintered body various parts are different.
(4) the balance of sintering of unbalanced cooling and rapid cooling, which after the alloy solidification, due to the diffusion process is difficult, if too fast cooling, WC content in supersaturated state, at this point, the microstructure could be WC original primary + + WC WC + Y solid solution, eutectic alloy composition is still a WC + Y, but the WC grain size.
3, the sintering process: the actual sintering process not only because of components to reach the ideal mixing and the imbalance caused by warming faster, also due to there are free carbon in the mixture, two tungsten carbide, or due to the effect of sintering furnace gas in the process of the sintered body composition change, which complicate the sintering process of organizational change.
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Carbon sintering characteristics: (1) left low eutectic temperature: 1280-1300 ℃, liquid temperature is low, the liquid phase number, the liquid phase composition is complicated, graphite may occur after cooling.
(2) the sintering characteristics of decarburization: eutectic temperature: 1370 ℃, a liquid temperature, liquid quantity less.
4, medium pressure sintering process of contraction, briquetting contraction can be roughly divided into three stages: diffusion, flow, rearrangement;
(1) due to temperature increase, atomic more activation on the surface of the powder particles, when the temperature reached 400-500 ℃, the bonding metal began to spread on the surface material along the free surface (diffusion), and with the temperature increase in volume diffusion (material within the crystal lattice of diffusion), in the space of WC particles gradually formed around the bonding metal mesh. When the temperature reaches 800-900 ℃, WC particle diffusion from the surface to volume diffusion, to shorten the distance between WC particles, shrink produce sintered body. Such as cobalt in mixture can uniformly surrounded the WC particles, the more obvious shrinkage.
(2) with the increase of temperature, bonding metal yield into plastic substances at the eutectic temperature, small plastic flow under stress to produce again, change the contact state between the powder particles, make carbide particles move and close, make the sintered body contraction.
(3) each carbide particles surface has a thin layer of liquid phase, as well as lubricant, reduce friction between the carbide, due to surface tension effect, make the carbide particles moving in the direction of more closely, rearrangement, make further density sintered body, carbide particles of the contact point between energy is higher, first dissolve in the liquid phase, and in other parts of the precipitation, and shortened the distance between carbide. Dissolving precipitation caused some carbide grain grew up along certain direction, so that the adjacent bonded carbide grain or aggregation, form a skeleton.
5, factors affecting shrinkage in the process of sintering:
Speed: (1) warming if heating up fast, is the highest shrinkage temperature range will be higher, or even move to the liquid temperature; Heating up too fast, there are a large number of coarse pores and bubble, this is due to the liquid phase in closed after the gas caused by the discharge channel.
(2) pressure piece of original pore: in inert atmosphere sintering briquetting, shrinkage is inversely proportional to the speed and density of briquette. So, alloy original porosity and density of briquette has nothing to do. When activated sintering atmosphere, porosity of briquetting is difficult to fully dense, so you must increase the density of briquette.
(3) grinding degree and the mixture of granularity: carbide particles of the thinner, the smaller the sintered body of single hole size, and capillary pressure of liquid is inversely proportional to the radius of the hole, at the same time, the center of the two carbide particles radius decreased along with the reducing of the particle size, and fine particles are easy to close. In addition, the bigger the specific surface of the powder and the solid phase diffusion speed and the coming of the liquid phase rearrangement precipitation and dissolution rate, the greater the therefore, the fine particles a contraction of low temperature, also can improve the shrinkage rate of solid phase sintering stage and shrinkage.
(4) : the mixture of cobalt content of cobalt content affects the liquid after shrinkage, cobalt content, the higher the sintering phasor of fluid, the more the greater the shrinkage rate, also can greatly promote the contraction of the second stage, because, this phase is the shrinkage mechanism of plastic flow.
(5) the carbon content, carbon content of the sintered body height, the number of the influence of the temperature of the liquid phase and liquid phase, therefore, the carbon content is bound to influence in the process of sintering shrinkage, the higher the carbon content and temperature of the eutectic point is reduced, is advantageous to the plastic flow, promote contraction speed, on the other hand, reduce the shrinkage rate. It should be pointed out that: oxidation serious material, is difficult to sintering density, it is more difficult than the lack of carbon material density.
(6) bonding metal wettability of carbide: wettability is refers to the liquid phase for solid phase infiltration ability. Carbide sintering, due to some reason make the wettability of liquid phase for solid phase bad, can make the sintered body contraction bad, cause pore, even "sweat" to discard. To improve the wettability can take the following measures:
A, raise the temperature; B, pure solid surface can improve the wettability (serious oxidation and sundry pollution is to eliminate the WC); C, vacuum conditions, because the vacuum conditions can purify the solid surface.
In addition, if improve the dissolution of solid in liquid read, changes in plants could also improve wettability.
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